In many modern digital transmission systems, digital transmitters modulate digital information onto radio frequency carrier signals. The modulated information signals are propagated to digital signal receivers where the digital signals may be demodulated and decoded.
For example, quadrature amplitude modulation (QAM), as detailed in International Telecommunications Union (ITU) recommendation ITU-T J.83B/SCTE DVS-031/DOCSIS, and vestigial side-band (VSB) modulation, as detailed in the Advanced Television Standard Committee (ATSC) standard A53/B, are two common digital modulation techniques that are used for cable and terrestrial digital television broadcast.
Invariably, propagation media for the signals introduce distortions in signals received at the receiver. Distortions may, for example, be introduced as a result of noise, phase shifts, signal attenuation, and multi-path interference. Imperfections at the transmitter may similarly introduce distortions in the transmitted signal. Ultimately, these distortions may manifest themselves as increased bit errors in the decoded digital signal at the receiver.
To reduce the overall bit error rate in the decoded signals, introduced distortions may be compensated in numerous ways. For example, to mitigate the effects of additive noise, the digital signals may include forward error correcting (FEC) codes. One or more equalizers may be used at the receiver to compensate for phase and amplitude distortions introduced into the transmitted signal by the propagation medium, such as multipath distortion.
For a terrestrial radio-frequency (RF) propagation medium, the characteristic of the propagation medium varies in dependence on a number of factors, including the location of the receiver. Accordingly, any equalizer at the receiver is typically adaptive and allows its parameters to be matched to the propagation medium at the receiver. The equalizer should also be able to compensate for variations in the propagation medium over time.
An example digital receiver including an adaptive equalizer is, for example, more particularly detailed in U.S. Pat. No. 6,418,164, the contents of which are hereby incorporated by reference. As disclosed, a conventional digital receiver may include a feed forward equalizer (FFE), a decision feedback equalizer (DFE), and a carrier recovery circuit, each designed to compensate for distortion in the received signal.
The FFE and DFE minimize multipath inter-symbol interference. Specifically, the FFE mitigates the effect of delayed echo multipath distortion (“pre-cursor”), while the DFE minimizes leading (“post cursor”) multipath interference. Error metrics between the demodulated signal and the reconstructed signal are used to provide closed loop adjustment of equalizer parameters. The carrier recovery circuit compensates for shifts in phase and/or of the carrier frequency of the broadcasted signal. The aim is to adjust simultaneously FFE and DFE equalizer and carrier recovery circuit parameters to compensate for the propagation medium distortion. A blind equalization technique using for example, the Constant Modulus Algorithm (CMA), may be used to adjust equalizer parameters.
Although very effective for channels having static characteristics, the disclosed receiver may not be able to track fast changes in the carrier frequency or phase. This is particularly acute when the receiver is used to receive VSB modulated signals, which are highly carrier dependent. Indeed, for VSB signals phase distortion affects real and complex values of the VSB constellation differently. Furthermore, a CMA will not be phase independent when applied to a signal modulated with a VSB constellation, as it is with a QAM modulation for example.
Accordingly, there is clearly a need for a receiver including a carrier recovery circuit that can track quick changes in carrier phase/frequency without otherwise impacting the overall performance of the receiver.